Um, I obviously weigh myself with clothes off before and after showers. I take warm showers, cooler than most people actually. Its baffling!
You could always do a test...
Put something that weights about 20lbs or so on the scale... like a pile of books or whatever... Record the weight
Then Take a shower... and weigh the same pile of books and see if the weight changes... you know the books didn't gain or lose weight so the weight should be the same...
If it's not the same you know that it's humidity altering the scale